The present invention relates in general to methods and circuits for reducing power consumed while an electronic system is turned off and, more particularly, to methods and circuits that discharge residual energy from the control terminal of a transistor, where the residual energy would otherwise cause the transistor to unnecessarily consume power while the transistor is switched off.
Many electronic systems rely on batteries to supply power to the systems"" circuitry. These systems include, for example, cellular telephones, pagers, radios, portable telephones, portable or remote computers, automobile electronics, and other systems. Because these systems"" energy sources are limited, designing circuits that efficiently consume and conserve power is crucial to increasing a system""s xe2x80x9cuseable time,xe2x80x9d which is the amount of time that the system can function on the limited battery power.
An operating cycle of a system may include an active mode, when the system is powered up, and an off mode, when the system is powered down. In addition, some system operating cycles include a standby mode, when only some of the system""s circuitry is powered up. Most devices consume more power when they are in an active mode, rather than in standby mode or off mode. For example, a cellular telephone consumes much more power during a call (i.e., active mode) than when the telephone is switched off or is in standby mode, waiting for an incoming or outgoing call. While off or in standby mode, certain circuits within the telephone are not actively functioning, and thus they desirably consume little or no power.
In reality, however, even when some of the system""s circuits are not actively functioning, they continue to draw some amount of energy from the battery. When prior art systems are switched off, residual energy exists on the control terminals (e.g., the bases) of various transistors, and it causes those transistors to slowly drain energy in the form of leakage current. For example, the DC-DC converters of some prior art portable devices may draw on the order of 100 microamps or more while the DC-DC converter is supposed to be switched off.
Prior art systems sometimes use a current source, during off mode, to provide a continuous, static current. When the current source is on, it draws energy from the transistor bases of some circuits (e.g., a DC-DC converter circuit), and the circuit power consumption decreases dramatically.
One problem with such prior art systems is that this continuous current still requires a certain amount of power consumption during the off mode. Accordingly, the device""s useable time is decreased.
Hence, there is a need for an apparatus and method that further reduce the amount of power consumed by a circuit when the circuit is switched off. Further needed is an apparatus and method for reducing off mode power consumption, which do not rely on using a current source to draw energy from a circuit""s transistors during off mode.